Various information appliances are used recently, and there is a mounting need for increasing the memory capacity in magnetic disk apparatuses, and high density recording is demanded more and more. In this trend, high reliability is needed together with high precision of rotation of disk and the bearing device for spindle motor of a magnetic disk apparatus is desired to be smaller in rotation deflection of shaft or surface deflection of disk in order to enhance the recording density. To meet such demands, instead of ball bearings, dynamic pressure fluid bearings of small vibrations have come to be used widely in these apparatuses.
A conventional spindle motor, an information recording and reproducing apparatus having such spindle motor, and a manufacturing method of spindle motor are explained below by referring to the accompanying drawings.
FIG. 38 and FIG. 39 are drawings explaining the principal mechanical parts of a spindle motor using a dynamic pressure fluid bearing widely used in information appliances including the apparatus for recording and reproducing information by making use of magnetism, and the principal mechanical parts of the information recording and reproducing apparatus using such spindle motor, and more specifically FIG. 38 is a schematic sectional view of principal structure of information recording and reproducing apparatus having spindle motor, and FIG. 39 is a schematic sectional view partially magnifying the area near the lubricant sump in the spindle motor using a dynamic pressure fluid bearing. In FIG. 38 and FIG. 39, components corresponding to the elements in FIG. 1A relating to a first exemplary embodiment of the invention described later are identified with same reference numerals as in FIG. 1A.
In FIG. 38, a rotating element 380 is composed of a columnar rotary shaft 382 which is a shaft member having a rotor hub 381 fixed as a rotating member, a rotor yoke 11 integrally coupled to the rotor hub 381, and a rotary magnet 12 magnetized in plural poles being affixed to the rotor yoke 11. On the other hand, a stator 16 is composed of an iron core 14 fixed on a base 10 and disposed oppositely to the inner circumference of the rotary magnet 12, and a coil 15 wound around the iron core 14. A bearing sleeve 7 which is a fixed side bearing on which the columnar rotary shaft 382 as a shaft member is rotatably fitted about the rotation center axis 1 is fixed to the base 10. Further, at the lower end of the bearing sleeve 7, there is a thrust plate 8a fixed to a thrust support plate 8, and the spherical lower end 832a of the rotary shaft 382 is supported in the thrust direction.
In the inner circumference of the bearing sleeve 7, a lubricant sump 383 of an annular recess is disposed closely to the upper opening end. The portion from the upper end of the lubricant sump 383 to the opening end of the bearing sleeve 7 forms a lubricant holding area (an excess oil region) 384 which is a cylindrical fluid holding portion of a specified length along the outer circumference of the rotary shaft 382.
The bearing gap between the rotary shaft 382 and bearing sleeve 7 is filled with a dynamic pressure lubricant 21 for generating a dynamic pressure, and on the surface of the thrust plate 8a facing the lower end 382a of the rotary shaft 382, a shallow groove is formed for obtaining a support force in the thrust direction by generating a dynamic pressure in the lubricant. On the outer circumference of the rotary shaft 382, first and second radial dynamic pressure generating grooves 382b, 382c of herringbone shape are formed upward from the lower end, having widths L1 and L2 in the axial direction, with the central distance between the widths L1 and L2 being L.
When the coil 15 of the stator 16 is excited by a driving current supplied through a drive and control circuit (not shown) of the information recording and reproducing apparatus, the rotary magnet 12 rotates together with the rotary shaft 382, thereby composing a spindle motor.
A disk board 385 on which a recording medium layer is formed is placed on the flange 381a of the rotor hub 381 of the spindle motor, and is pressed and fixed to the flange 381a of the rotor hub 381 by other end of a pressure spring 386 of which one end is fixed in a groove formed in the rotary shaft 382. By integrally fixing the disk board 385 together with the rotating element 380 comprising the rotor hub 381 and rotary magnet 12, the disk board 385 is rotated along with rotation of the spindle motor, and by recording and reproducing in the disk board 385 by a known method, by using a magnetic head (not shown) mounted on a slider, or an optical pickup (not shown) having an objective lens for focusing the light, an information recording and reproducing apparatus is composed.
When the rotary shaft 382 is rotated by the spindle motor having such dynamic pressure fluid bearing, the dynamic pressure lubricant 21 in the bearing gap is sucked into the center of the radial dynamic pressure generating grooves 382b, 382c, and a dynamic pressure for obtaining a supporting force in the radial direction is generated. By such dynamic pressure fluid bearing, the rotary shaft 382 during rotation is kept free from contact with the inner circumference of the bearing sleeve 7. Since the rotary shaft 382 rotates without contacting with the bearing sleeve 7, rotation deflection of shaft or surface deflection of disk board 385 can be decreased, and the reliability of the information recording and reproducing apparatus can be enhanced together with the rotation precision of the disk board.
In the spindle motor having such dynamic pressure fluid bearing, the dynamic pressure lubricant 21 may flow out from the opening in the bearing sleeve 7 during rotation, or the dynamic pressure lubricant 21 may overflow in the assembling process, or the dynamic pressure lubricant 21 may ooze out due to thermal expansion of the dynamic pressure lubricant 21 by temperature rise. To avoid such troubles of flowing-out or oozing-out of the dynamic pressure lubricant 21, and also to prevent overflow of dynamic pressure lubricant 21 due to lack of capacity of the lubricant sump 383, a technology is proposed, for example, in Japanese Patent Publication No. JP2000-121986. According to this proposal, as shown in FIG. 39, a lubricant sump 383 of annular groove is provided in the inner circumference of the bearing sleeve 7, and the upward groove end forming the lubricant sump 383, that is, the upper end 383a of the end face at the opening end side of the bearing sleeve 7 is inclined in a reverse taper. In this proposed method, the opening width in the axial direction of the lubricant sump 383 is smaller than the width of the bottom of the lubricant sump 383, and the length in the axial direction of the lubricant holding part (excess oil region) 384 is substantially increased by the corresponding portion. That is, the upper end 383a of the lubricant sump 383 is formed in a reverse taper to increase the inner volume of the lubricant sump 383 and the lubricant holding portion (excess oil region) 384 is extended, and thereby it is intended to avoid troubles of flowing-out or oozing-out of the dynamic pressure lubricant 21.
The grooves for generating the dynamic pressure of the spindle motor having such dynamic pressure fluid bearing can be formed by plastic processing method such as etching, shot blasting, shot peening, or ball rolling. For example, as disclosed in Japanese Patent Publication No. JP7-164251, using a downsized rolling die device, an inexpensive technology for processing grooves for generating dynamic pressure in the rotary shaft is proposed, in which the portion not forming dynamic pressure generating grooves is preliminarily formed by cutting process or the like, and then herringbone grooves for generating dynamic pressure are formed by rolling in the rotary shaft.
However, in the conventional configuration of the information recording and reproducing apparatus having the spindle motor or the disk board 385 mounted on the flange 381a of the rotor hub 381 of the spindle motor, the rotor hub 381 is fixed to the rotary shaft 382, and further the disk board 385 is held in the flange 381a of the rotor hub 381, and therefore it is not only hard to mount the flange 381a on the rotary shaft 382 by precisely crossing the disk mounting face of the flange 381a or the recording face of the disk board 385 orthogonally to the rotation center axis 1 of the rotary shaft 382, but also hard to match the center of the flange 381a or the disk board 385 precisely with the rotation center axis 1.
Due to such deviation in angle formed between the disk mounting face of the flange 381a or the recording face of the disk board 385 and the axial center of the rotary shaft 382, when the disk board 385 is rotated, the recording face rotates with a certain inclination, and the position of the recording face of the disk board 385 fluctuates, that is, a phenomenon of surface deflection occurs. Also due to deviation in position of the center of the flange 381a or disk board 385 from the rotation center axis, when the disk board 385 is rotated, the position fluctuates in a direction parallel to this surface, that is, an axial center deflection occurs.
Yet, if the temperature is raised owing to the environments of use of the information recording and reproducing apparatus or temperature rise inside the apparatus, the oil viscosity of the dynamic pressure lubricant 21 drops, and the bearing rigidity of the radial bearing declines, and therefore at the time of recording or reproducing of the apparatus, the surface deflection of the disk board increases.
In actual use, the surface deflection and axial center deflection appear in a combined form, and the recording density of the disk board 385 must be determined by including a tolerance in consideration of such fluctuations, and hence there was a limit in enhancing the recording density.
To suppress the surface deflection and axial center deflection of the disk board 385 during rotation, it is required to enhance the processing precision and assembling precision of these constituent members, and it means that increase in the apparatus cost is inevitable. It further requires a tightening part for fixing the disk board 385 to the flange 381a of the rotor hub 381, which also causes to increase the cost.
To maintain a high precision of rotation of the disk, it is required to extend the shaft diameter and shaft length of the rotary shaft for composing the radial bearing and thrust bearing, and the space occupied by the rotary shaft increases, and it is hard to reduce in size and thickness. Further, a space for tightening the disk board 385 to the flange 381a of the rotor hub 381 is required in the upper part of the disk board 385, which makes it hard to reduce in thickness, too.
It is also necessary to enhance the moment axial rigidity to the disturbance moment applied on the rotating element 380, and the following four methods are generally known as the means for enhancing the moment axial rigidity.
(1) To enhance the bearing rigidity to the side pressure in the radial bearing.
(2) To widen the gap L between the first and second radial dynamic pressure generating grooves 382b, 382c of the radial bearing.
(3) To suppress lift in the thrust bearing by increasing the magnetic attraction applied to the thrust bearing.
(4) To raise the oil viscosity.
These methods, however, have own problems to be solved as discussed below.
(1) To enhance the bearing rigidity to the side pressure in the radial bearing, the gap in the radial bearing is narrowed, or the widths L1 and L2 in the axial direction of the first and second radial dynamic pressure generating grooves 382b, 382c are increased. But to narrow the gap of the radial bearing, it is required to process the radial bearing parts at very high precision, and there is a limit in processing precision of parts. Therefore, the gap in the radial bearing cannot be narrowed. Or if the widths L1 and L2 in the axial direction are increased, the spindle motor cannot be designed in a reduced thickness.
(2) To design a thin spindle motor, it is not allowed to widen the gap L of the first and second radial dynamic pressure generating grooves 382b, 382c of the radial bearing.
(3) Since the facing area of the thrust bearing is small, the thrust proof load is small, and metal contact may occur when the oil viscosity is lowered at high temperature, in particular.
(4) When the oil viscosity is raised, the bearing loss increases extremely in low temperature region.
In the conventional configuration of the spindle motor in the information recording and reproducing apparatus having a lubricant sump of a wedge ring form in the bearing sleeve shown in FIG. 38, it is hard to observe the oil distribution state of the dynamic pressure lubricant, and it is possible that an excessive oil may be supplied from the bearing sleeve open side end as the dynamic pressure lubricant 21 flows over the lubricant sump 383 as shown in FIG. 39. In particular, oil feed amount control is more difficult in a smaller information recording and reproducing apparatus. As indicated by arrow in FIG. 40, an excessive lubricant 401 oozes out from the bearing sleeve 7, or oil drops of the excessive lubricant 401 oozing out are provided with a centrifugal force by rotation of the rotary disk, and splash out of the motor to stick to the rotary disk surface, and the recorded data may be damaged, and hence there was a problem in reliability.
Dynamic pressure generating grooves may be processed, as mentioned above, by etching, shot blasting, shot peening, or ball rolling, but if the entire sleeve is made of a hard material, the grooves for generating dynamic pressure must be formed by etching or shot blasting process, and the processing cost is high. On the other hand, when the entire sleeve is made of a soft material, the grooves for generating dynamic pressure can be processed easily, and ball rolling or other plastic processing of low processing cost may be easily applied. However, when deburring the inner circumference or correcting the coaxiality of inside diameter and outside diameter in the finishing process after ball rolling, the outer circumference may be ruined, and the inner circumference cannot be processed on the basis of the outside diameter, and hence the coaxiality of inside and outside diameters cannot be enhanced. Or when forming the grooves for generating dynamic pressure in the inner circumference of the bearing sleeve to compose the radial bearing, a bearing sleeve having dynamic pressure generating grooves formed at high precision is required, and the cost becomes higher.
In particular, when the rotary shaft is not a simple columnar form, but is integrated with a rotor hub for fixing the disk board, a special holding tool is needed for forming and processing the grooves for generating dynamic pressure in the rotary shaft, and the processing device becomes larger and complicated, and the processing cost is raised.